For many processes, particularly in the petroleum processing industry, it is necessary to provide hydrogen as a portion of the charge. These hydrogen-consuming processes are usually high pressure processes such as hydrogenation or hydrocracking and they are usually effected in the presence of a catalyst. Ordinary operation of a refinery does not produce sufficient hydrogen to supply these processes; and as a result, an outside source of hydrogen must be provided. Additionally, catalytically effected processes are usually quite sensitive to catalyst poisons; and it is necessary that the hydrogen produced be quite free from even small quantities of impurities that may affect the activity of the catalyst.
Hydrogen may be formed in a process where hydrocarbons are partially oxidized with oxygen or water to produce carbon monoxide and hydrogen. One typical process of this nature is known as the steamhydrocarbon reforming process. It is effected by oxidizing light hydrocarbons and reducing water in the presence of a catalyst to produce high yields of hydrogen and carbon monoxide. The process is favored by high temperatures and it is usually performed at temperatures in the neighborhood of 1400.degree.-1500.degree.F and at pressures of about 200 psig. Higher pressures have such an adverse effect on the equilibrium of the reaction that they must be avoided.
The high yield of hydrogen can be increased if the gas from the partial oxidation reaction is subjected to a process that is known as the carbon monoxide shift conversion reaction. The gases from the partial oxidation process are mixed with water and passed into contact with a catalyst that promotes the oxidation of carbon monoxide and reduction of water to produce hydrogen and carbon dioxide. This reaction is benefited by lower temperatures, and it is usually effected at temperatures in the neighborhood of 450.degree.F. The reaction is frequently performed in two stages, the first stage at higher temperatures with a less active catalyst and the final stage at about 450.degree.F with a more active catalyst.
To produce hydrogen of sufficient purity for a catalytic hydrogen-consuming process, it is necessary to remove the large volume of carbon dioxide in the effluent from the carbon monoxide shift conversion reaction. Carbon dioxide is an acidic gas, and it can be removed to low levels by being absorbed in a suitable solvent, preferably a basic solvent that has both chemical and physical activity. A particularly useful solvent of this type disclosed in U.S. Pat. No. 3,347,621 issued to Papadopoulos is one containing diisopropanolamine, a cyclotetramethylene sulfone and water. Other absorbents that are used include aqueous monoethanol amine and hot potassium carbonate solutions.
The gas recovered from such an absorption process is very low in carbon oxides and very rich in hydrogen, but even this gas contains too much carbon monoxide for use in modern catalytic hydrogen-consuming processes since catalysts employed in such hydrogen-consuming processes are generally sensitive to carbon monoxide poisoning.
Accordingly, the absorber effluent gas may be subjected to a methanation reaction wherein the absorber effluent is contacted with a catalyst at conditions suitable for reacting carbon oxides with hydrogen to produce methane and water, both of which are relatively inert impurities with respect to the catalysts usually employed in hydrogen-consuming processes. The gas from the methanation reaction zone is suitable for use in high pressure catalytic hydrogen-consuming processes, and it is generally compressed to the high pressure that is typical of hydrogen-consuming processes and introduced into the hydrogen-consuming process as a portion of the charge. All of the foregoing reactions, and this particular combination of these reactions, are known to the prior art.